Cellular phone blockers have been available since 1998. These blockers are used for blocking inbound and outbound cellular phone calls. There are two general methods to block a call: interfere by transmitting at the cellular frequency to be blocked; and by establishing an electronic filter to fool mobile phones that the reverse link is not being responded to, i.e., make it seem that the mobile unit is out of range of the base station.
There are many reasons for blocking cellular transmission. Preventing noise in an area where the cellular phone would be annoying, such as restaurants, theatres, halls, hospitals and libraries. Further, there are some environments were cellular transmission may threaten sensitive instruments such as in hospitals. The literature reports that some Latin American banks routinely block cellular transmission so that spotters within banks cannot communicate to outside henchmen identifying a bank patron who has made a sizable cash withdrawal.
However, radio frequency jamming is illegal in most industrial nations with two exceptions: Israel and Japan. Exceptions are available in other countries for narrow reasons such as public safety and national security. For example, in an environment such as a hospital or airplane where radio transmission may affect instruments critical to safety, blocking of wireless transmission may be granted an exception to a law banning such jamming. In a military, security or police situation, where it is critical to block espionage, terrorist or criminal communication, exceptions may be granted. A manufacturer of cell jamming equipment, Intelligence Support Group (ISG), Ltd., based in China Lake, Calif., sells its equipment only for export, to the military, or to those law enforcement agencies exempted from FCC rules.
ISG describes its product, Cellular Call Blocker, Model: CELL-B, as a device that allows blocking of cellular calls by authorized law enforcement and government agencies by overriding the cellular carriers' control channel signal with a signal from the CELL-B. This forces mobile phones in the target area to a roam mode, thus only responding to the CELL-B control channel signal. The overiding causes all cellular phones in the target area to be jammed.
U.S. patent application Ser. No. 200010036821 filed by Gainsboro et al. entitled, “Computer-Based Method and Apparatus for Controlling, Monitoring, Recording, and Reporting Wireless Communications,” (herein, Gainsboro) describes a microprocessor-based method and apparatus for controlling wireless communications (via cellular radiotelephone or some other known means) into or out of a controlled, institutional environment, such as a prison, military base, hospital, school, business or government organization. A radiotelephone interface is provided, with apparatus for detecting and controlling wireless transmissions either by a caller from within the institution or by a caller from outside the institution. The identification of an unauthorized wireless transmission is accomplished through the detection of certain identifying signals, i.e. NPA/JNXX/ESN, or, alternatively, via voice recognition. The system monitors wireless transmissions, tests signals for approval, and enables or prevents the transmissions via jamming or other means. Gainsboro, however, does not teach how such transmissions are to be jammed or provide a means for limiting the jamming transmission so as to minimize interference with non-targeted terminals.
What is desirable is a cellular phone blocker that is selective (i.e., intelligent) in implementation wherein the device operates only when a targeted mobile terminal is attempting to place a call while in a limited area. This way, non-targeted cellular phones are operational within the limited area as long as the targeted cell phone is quiescent.